The invention relates to a graded layer or zone at the surface of fused silica for reducing its sodium ion conductivity, and to a method of treating fused silica to produce such zone. The invention is particularly useful to slow down sodium ion diffusion through the arc tube walls of sodium containing metal halide lamps, and is also useful in other applications where sodium diffusion is undesirable.
Metal halide lamps generally contain a filling of mercury and light-emitting metals including sodium in the form of halides, commonly the iodide, in fused silica envelopes. The mobility of the sodium ion is such that the fused silica or quartz is relatively porous to it, and during lamp operation, sodium will pass from the hot arc plasma through the fused silica wall to the cooler region between the arc tube and the outer jacket or envelope of the lamp and condense on the outer envelope and parts. The lost sodium is unavailable to the discharge and can no longer contribute its characteristic emission so that the light output gradually diminishes and the color shifts from white towards blue. The arc becomes more constricted and, in a horizontally operating lamp particularly, may bow against the arc tube wall and soften it. Also loss of sodium causes the operating voltage of the lamp to increase and it may rise to the point where the arc can no longer be sustained by the ballast. At this point the life of the lamp is ended.
In the past, several different types of coatings on fused silica have been tried to reduce sodium ion diffusion through the envelope wall of metal halide lamps. One barrier proposed comprised a layer of zirconium oxide upon the inner surface of the arc tube to inhibit sodium diffusion and a second layer of a refractory oxide such as calcium, magnesium or aluminum oxide to protect the zirconium oxide layer from the attack of the arc stream. Such coatings are difficult to apply and have not found acceptance.
Sputtered alumina coatings have been tried but have been generally unsuccessful, probably because the coatings were either too thin to be effective or else too thick to remain continuous and adherent. The large difference in thermal expansion between fused silica and metal oxide coatings causes thick coatings to eventually tear apart under thermal cycling.
The object of the invention is to provide an improved barrier for reducing sodium ion conductivity and the rate of sodium ion diffusion in fused silica, and a novel method for forming such barrier.